This invention relates to forming thermoplastic interlayer sheeting and more particularly to sheeting and a method for its formation which is used to form discrete, shaped blanks suitable for laminated safety glass windshields.
Thin sheet usually formed of plasticized polyvinyl butyral is well known as an interlayer in laminated safety glass finding application in windows such as penetration-resistant automobile windshields. A very well known problem in view of many patents dealing with it since the first in the United States issued in 1952, is the performance deficiency in the interlayer when used in a specially curved and angled windshield. More specifically, sheet in such applications has traditionally been shaped in association with glass laminating downstream of sheet forming by stretching into a shape to match the desired windshield configuration. Special problems arise when the interlayer has a colored band along one margin which is intended to reduce glare from the sun when in place in the windshield. The band is usually graduated in color intensity with the greatest color along the upper peripheral portion of the windshield which then gradually diminishes to an almost imperceptible cutoff line at the lower edge of the band. When such gradient band windshield has horizontal and vertical curvature, as is usual with modern wrap-around automobile windshields, it has been necessary to stretch each sheet section to an accurate shape before associating it with the glass so that after lamination the cut-off line of the band is parallel to the upper edge of the windshield. This amount of initial stretching compensates for the initial lack of curvature of the color band which if unstretched would produce a cut-off line not parallel to the upper edge of a curved wrap-around windshield.
As well delineated in the prior art, for example col. 2, lines 3-28 of U.S. Pat. No. 4,244,997, uneven stretching of sheet after its formation adversely affects its subsequent performance in a windshield. More specifically, differential stretching of sheet of initial constant thickness inherently results in localized thickness reduction which translates to variable impact resistance in the windshield, such resistance being lower in the area of reduced thickness. It would be desirable to provide a system for forming interlayer windshield blanks reducing or eliminating such non-uniform thickness shortcomings of the prior art.